Itch is a primary symptom of many allergic conditions, and severely affects the quality of life and productivity. Primary sensory neurons residing in the trigeminal ganglia and dorsal root ganglia (DRG) play an essential role in the generation of allergic itch. These neurons detect endogenous itch-inducing molecules (pruritogens) via a group of itch receptors on their peripheral axons in the skin or mucosal membrane tissues, and transmit signals to the spinal cord via their central axons. In contrast to the well-studied histamine pathway, histamine-independent itch pathways in allergic itch remain poorly defined. Previously, we identified several novel itch receptors within a large family of G-protein?coupled receptors called Mrgprs. We found that several Mrgprs recognize distinct pruritogens, and mediate histamine-independent itch. Our latest results indicate that Mrgprs are required for mast cell-mediated allergic itch. This proposal aims to uncover the underlying mechanisms. We will test whether Mrgprs mediate the interaction between mast cells and primary sensory fibers in allergic itch using molecular, genetic and imaging approaches in Aim 1. Furthermore, we will identify the endogenous Mrgpr ligands released by mast cells upon degranulation in Aim 2. In Aim 3, we seek to translate our discoveries from mice to humans. Human Mrgprs do not form clear orthologous pairs with mouse Mrgprs. Their role in itch is therefore unclear. Based on our preliminary data, we hypothesize that human MrgprX1 mediates neuronal and behavioral response to mast cell-mediated allergy, which will be tested in Aim 3. These studies will reveal the neural basis underlying allergic itch and will have a significant impact on both the study of itch pathogenesis and the clinical treatment of itch.